Levothyroxine is a thyroid hormone product that is commercially available in II different dose strengths, of which only 5 strengths are permissible in the hospital formulary. The purpose of this prospective, randomized, controlled trial is to determine whether limiting dosage strengths of levothyroxine affect physicians' ability to effectively manage patients. This trial will assign participating endocrinologists from the NICHD and NIDDK outpatient clinics to restrictive (25, 50, 100, 125, and 150 microgram dosage formulations permitted) or non-restrictive (25, 50,75, 88, 100, 112, 125, 150, 175, 200, and 300 microgram dosage formulations permitted) levothyroxine prescribing groups. Success in achieving therapeutic objectives as measured by thyroid function studies and clinic visits, medication distribution accuracy, and inventory cost will be compared statistically between groups. Preliminary results from 241 patients of 33 endocrinologists suggest that the differences in therapeutic success between restricted and non- restricted thyroid patients were not clinically significant. The compliance, frequency of thyroid function tests, clinical visits, and medication errors were also similar. The restricted formulary however, was significantly more often associated with complex levothyroxine dosing regimens. Further, the inventory and prescription costs were slightly lower with use of the non-restrictive formularly system. Presently, 890 patients are being followed in this protocol.